Heart-felt Apparel
Wearables that detect
your
heart beat and externalize it as pulses of light. Sensors read the
wearer's
EKG and produce flashes of light in time with his/her own heart.
Wearing the shirt gives an intense feeling of life and rhythm, while
at the same time reminding you of your electrical and mechanical
roots. It's truly amazing how much the heart responds to social
interaction and yet is almost entirely ignored as it meters our life.

Pulsing with every beat, the Heart-felt Wristband also changes color
from blue to red as your heart rate slows down or speeds up. Modify
it to be worn as a wristband, belt buckle or even as
a medallion... put a little heart into your bling!

Here is a schematic of the
electrocardiogram (a.k.a. ECG or EKG) signal, identifying
the physical events that correspond to each of the
observed electrical deflections as recorded from two
electrodes places on right and left sides of the
chest.

Amplification:

The first step is to amplify and buffer signal into a
range that can be detected with common circuitry.
Below is a common circuit with a driven right leg (DRL),
commonly used to reduce noise using common-mode
rejection.

Here is a similar amplifier circuit soldered
together. Note that this is just the analog portion of
the circuit, amplifying the signal. The circuit works
quite well, athough can be overrun by really heavy
noise created by other electronics (especially
el-wire).

Detection:
Once the signal is amplified, we must
devise a method to detect the beats. This is easy if
the signal is exactly like the schematic above, but
consider the following complications:

(1) Electrodes are reversed and the large peak is
negative.

(2) The signal size can vary by a factor of 10
depending on individual differences and electrode
placement.

To address these considerations, I did the
following:

(1) Rectify the signal, making all peaks positive
(above the baseline), turning the red signal into the
blue signal seen here. This leads to 2 peaks in rapid
succession, but this can easily be dealt with.

(2) Detect the peaks using a "leaky peak detector".
Whenever the EKG hits the threshold (green) it detects
a beat and pushes the threshold up to the highest
value of the EKG. After the peak, the threshold then
slowly decays back down until another beat pushes it
up. With an appropriate timescale for decay, this
method ensures that any beat that is above the noise,
no matter how large or small, will be detected.

I initially performed these operations using all
analog circuitry (left). A microcontroller is much
simpler (right).

Alternative:
Off-the-shelf Wireless Solutions!

More recently, several
off-the-shelf solutions have come onto the market from
companies selling exercise straps. You buy a strap,
put it on, and it takes care of all the heart beat
detection and wirelessly sends it to a specialized
receiver chip.

AMAZING!

Pulse detecting exercise straps are available from
various companies, including
Polar, Garmin, Sunto, and others. Receiver chips are
available from
sparkfun.com

The transmitter strap is set to go. Just follow
the instructions: Get it wet and put it on.
The receiver requires some minimal setup diagrammed below.

There are two outputs from the RMCM01 that display
a pulse for each heartbeat. The "coded" output works
with transmitters like the wearlink+ that send a
device ID with each signal. The coded signal permits multiple
users in close proximity to be accurately tracked by
their respective receivers. The non-coded output
displays all detected heartbeats from all nearby
transmitters including those sent by older Polar
straps that did not have a coded feature.

Complications with this Approach:

(1) The wireless signal is transmitted at 5.5kHz, which
can get interference from some electronics, such as
el-wire/panel. Other companies' products (Garmin, Sunto),
may deal with this better.

(2) The Polar strap
generally works very well. I found that over time,
however, it can begin it to lose effectiveness, at
which point you may need to buy a replacement strap
for ~$15. For my own
purposes, I switched to using a gel electrode attached
to the transmitter via snaps on a homemade strap. I
like the
Kendall 7305 Biotac Ultra Foam Electrodes because they
don't damage your skin the way 3M does, but get a
good signal and can be reused several times.